Single pass prereacted grain and method of making

ABSTRACT

A FULLY RECRYSTALLIZED, SINGLE PASS REFRACTORRY GRAIN IS MADE BY ADMIXING A FINELY DIVIDE WITH FINELY DIVIDED LOW SUCH AS MAGNESIUM HYDROXIDE WITH FINELY DIVIDED LOW SILICA CHROME ORE AND FIRING THIS ADMIXTURE, FOR EXAMPLE IN A ROTARY KILN, WITHOUT PRIOR COMPACTION, TO A TERMPERATURE OF AT LEAST 1975*C., THE MATERIAL BEING EXPOSED TO THAT TEMPERATURE FOR AT LEAST 10 MINUTES.

Patented June 18, 1974 3,817,765 SINGLE PASS PREREACTED GRAIN AND METHODOF MAKING William R. Alder, Fremont, Califi, assignor to Kaiser Aluminum& Chemical Corporation, Oakland, Calif. No Drawing. Filed June 19, 1972,Ser. No. 264,323

Int. Cl. C04b 35/12 US. Cl. 106-59 6 Claims ABSTRACT OF THE DISCLOSURE Afully recrystallized, single pass refractory grain is made by admixing afinely divided MgO-yielding material such as magnesium hydroxide withfinely divided low silica chrome ore and firing this admixture, forexample in a rotary kiln, without prior compaction, to a temperature ofat least 1975 C., the material being exposed to that temperature for atleast 10 minutes.

BACKGROUND OF THE INVENTION This invention concerns refractory grain,and more particularly a method of making such grain by reacting lowsilica chrome ore and an MgO-yielding material.

It has long been known to make refractory shapes from granularadmixtures of chrome ore and a magnesia material such as magnesite orpericlase. It is also known to make refractory shapes from grain made byprereacting these two materials, for example as disclosed in U.S. Pat.2,775,525 to Austin et al.

In making prereacted grain from chrome ore and magnesia, it is known tocalcine the MgO-yielding material, for example magnesium hydroxide, at atemperature in the neighborhood of 1000 C. to produce an activemagnesia, to admix this active magnesia with the chrome ore, to compact,for example by briquetting, this admixture, and then to fire thecompacts, for example in a rotary kiln, to produce dense refractorymaterial which can be crushed to the desired grain sizing for makingrefractory products such as bricks.

Obviously, this so-called double burn method of making prereactedrefractory grain is more expensive than making such grain in a singlefiring. However, it has been found in previous attempts to produceprereacted grain without calcination of the MgO-yielding material, andparticularly without compaction of the chrome and magnesia admixture,that the desired properties of low porosity and good recrystallizationof the grain are not achieved.

In addition, in recent years the trend in refractories technology hasbeen toward lower silica contents; hence, the use of beneficiated, lowsilica chrome concentrates in place of raw chrome ore. The use of thesehigher purity, low silica materials, however, makes it even moredifficult to achieve low porosities in prereacted grain made frommagnesia and chrome.

The present invention provides a method of producing dense, completelyrecrystallized refractory grain directly from chrome ore, and moreparticularly from low silica chrome concentrates, and MgO-yieldingmaterial in a single fire process without the necessity of compactingthe raw materials.

SUMMARY OF THE INVENTION According to the present invention, a sinteredprereacted refractory grain containing less than 2% SiO; is

made from MgO-yielding material and chrome ore by (1) intimatelyadmixing from 20% to by weight of MgO-yielding material, calculated asMgO, substantially all of said MgO-yielding material being less than 44microns in size and having a median particule size of less than 4microns, said MgO-yielding material containing less than 0.5% SiO on theignited basis, with from 80% to 20% by weight of chrome ore at least 95%of which passes a 325 mesh screen (i.e., is finer than 44 microns) and50% of which is finer than 10 microns, said chrome ore containing lessthan 3% SiO and (2) subjecting said admixture without prior compactionto a heat treatment wherein the admixture is exposed to a temperature ofat least 1975 C. for at least 10 minutes.

The resulting product is a fully recrystallized, uniform refractorygrain of periclase solid solution crystallites with exsolvedchromium-containing spinels within the periclase crystallites andcontaining less than 2% SiO The grain is substantially free of any ofthe original chrome ore in unreacted form, and, in a preferred form, hasa total porosity of less than 10 volume percent.

DETAILED DESCRIPTION The MgO-yielding material may be any such material,for example brucite or magnesite, but a preferred material is magnesiumhydroxide, most preferably magnesium hydroxide produced by reactingCaO-containing material such as calcined limestone or calcined dolomitewith brine, for example sea water, containing magnesium values insolution. The resulting precipitated magnesium hydroxide (Mg(OH) is ofthe desired fine particle size for use in this invention, the medianparticle or agglorncrate size being about 4 microns, and substantiallyall the particles or agglomerates being less than 44 microns in size. Asindicated, the so-called particles of precipitated magnesium hydroxideare actually agglomerates of finer crystallites, the ultimatecrystallite size being about 0.2 microns. However, in any case, nomatter what MgO- yielding material is used, at least must be finer than325 mesh, and it will have a median particle size of about 4 microns.The MgO-yielding material will contain at least and preferably 98% ormore, MgO on the ignited basis, the remainder being normal impurities.The chrome ore can be any such material, but will generally be of theso-called refractory grade. Such chrome ores are found in the Masinlocregion of the Philippines, in the Transvaal and other regions of Africa,in Turkey, and elsewhere. To achieve the required SiO content of lessthan 3% in the chrome ore, so-called chrome concentrates will generallybe used, these concentrates being chrome ore material which has beenbeneficiated to remove silicate materials. Thus, as used in thisspecification, the term chrome ore includes chrome concentrates. Ofcourse, a naturally occurring chrome ore of the requisite SiO contentcan be used without beneficiation. In any case, it is essential that thechrome ore be ground so that at least 95 passes a 325 mesh screen, andthat it has a median particle size of less than 10 microns. It will beunderstood that the finer the chrome particles, the better the resultsof the invention. Milling of the chrome ore, and of the MgO-yieldingmaterial where necessary, can be carried out in any suitable apparatus,for example, a vibratory energy mill such as a Sweco mill. A preferredmethod of milling is wet milling in a ball mill, for example using ironballs.

A particularly preferred method is to admix the MgO- yielding materialand the chrome ore and mill them together until both have the requisitefine particle size and are thoroughly interdispersed. It is an advantageof this invention that magnesium hydroxide can be much more intimatelyinterdispersed with the chrome ore than can the calcined magnesia usedin the doubleburn process.

The admixed raw materials are then charged, without any compaction, intoa kiln, for example a rotary kiln, where they are exposed to a peaktemperature of at least 1975 C., and preferably at least 2000 C. It isnecessary that the material be exposed to temperatures of at least 1975C. for at least 10 minutes. It has been found that shorter exposuretimes result in insuificiently reacted and nonuniform grain.

During firing, the spinel constituents of the chrome ore are dissolvedin the MgO, which is extensively recrystallized. At the peak firingtemperatures, substantially all the chromium spinel material isdissolved in MgO. However, upon cooling of the refractory material belowthe peak firing temperature, chromium-containing spinels exsolve fromthe MgO, appearing as dendrites in the MgO crystallites. The silicatematerials originally present in the chrome ore appear as a minorintergranular phase between the MgO crystallites. A very fewintergranular spinel crystallites are also present.

EXAMPLE Magnesium hydroxide was produced by reacting calcined dolomitewith sea water containing dissolved magnesium sulfate and magnesiumchloride. This magnesium hydroxide shows, on the ignited basis, thefollowing typical chemical analysis: 1.2% CaO, 0.3% Si 0.1% A1 0 0.2% FeO and (by difference) 98.2% MgO, all percentages being by weight. Allthe particles of Mg(OH) were smaller than 44 microns, and the medianparticle size was 4 microns.

Masinloc chrome ore concentrates were charged to a steel ball mill withsteel balls and dry milled to a median particle size of 9.3 microns. 95%of the milled product was finer than 44 microns. The chrome concentratesshowed the following typical chemical analysis: 18.7% MgO, 0.4% CaO,2.6% S 29.4% A1 0 13.9% R20, and 35.0% Cr O all percentages being byweight.

The milled chrome concentrates and damp magnesium hydroxide filter cakewere charged to a pug mill in the proportion of 75 parts by weight, drybasis, magnesium hydroxide to 48 parts by Weight chrome concentrates.The

damp Mg(OH) filter cake contained equal parts by to 60% by weight MgO.After leaving the pug mill, the

wet admixture was conveyed by means of a screw conveyor, where furthermixing took place, to a rotary kiln. The material was fired to a peaktemperature of 2000 C., the residence time of the material in the kilnat temperatures above 1975 C. being about minutes.

The resulting refractory grain showed the following chemical analysis:1.5% SiO 7.1% Fe O 13.6% A1 0 15.2% Cr O 0.8% CaO, and (by difference)61.8% MgO, all percentages being by weight. Petrographic examination ofthe grain showed it to be a thoroughly uniform periclase solidsolution-spinel with minor silicate composition, displaying a strongpericlase-chrome reaction. The periclase solid solution crystal sizeranged from 60 to 120 microns and averaged about 80 microns. The veryfew irregularly distributed intergranular spinel crystals averaged aboutmicrons in size. The inert and well reacted character of the grain isillustrated by the diificulty experienced in putting the material intosolution for purposes of running the wet chemical analysis. The poros 4ity of the grain determined by ASTM method C-493- was about 9 volumepercent.

The grain of the preceding example is to be compared with grain made insimilar fashion except that only about of the chrome ore concentrateswere finer than 44 microns (i.e., passed a 325 mesh screen). Thiscomparison grain had a similar chemical composition to that of theexample, but its porosity was about 15%. Microstructural examinationshowed it to be much less well reacted than the grain of the example.The crystallite size of the comparison grain ranged from 30 to 150microns, with an average of about 60 microns. The greater variability ofthis comparison grain is thus evident. Although the comparison graincontained periclase crystallites with exsolution chrome, it alsocontained considerable unreacted chrome material. In general, thecomparison grain was not as well bonded as the grain of the example.

The grain of the example can also be compared with a double pass grainmade from the same raw materials. The magnesium hydroxide was calcinedto a temperature of 950 C. to produce an active magnesia, and admixedwith the chrome concentrates ground to an average particle size of 10microns, over 95% of the chrome concentrates being less than 44 micronsin size. The dry admixture was compacted in a roll-type press at anequivalent pressure of 35 tons per square inch. The compacts so formedwere charged to a rotary kiln where they were exposed to a peaktemperature of 1975 C. Microscopic examination showed the resultinggrain to be made up of very uniform periclase solid solution-spinelcrystallites with minor silicate, and having an average crystallite sizeof 70 microns. The porosity was about 10%. In other words, even thedouble burn grain made by the more expensive process is not as wellreacted as the grain made according to the present invention.

In the specification and claims, percentages and parts are by weightunless otherwise indicated, except that porosites are expressed involume percent. Mesh sizes referred to herein are Tyler standard screensizes which are defined in Chemical Engineers Handbook, John H. Perry,Editor-in-Chief, Third Edition, 1950, published by Mc- Graw Hill BookCompany, at page 963. For example, a 100 mesh screen opening correspondsto 147 microns, and 200 mesh to 74 microns. Analyses of mineralcomponents are reported in the usual manner, expressed as simple oxides,e.g., MgO, SiO although the components may actually be present invarious combinations, e.g., as a magnesium silicate.

Having described the invention, what is claimed is:

1. Method of making prereacted sintered refractory grain containing lessthan 2% Si0 from MgO-yielding material and chrome ore comprising (1)intimately admixing from 20% to by weight of MgO-yielding material,calculated as MgO, substantially all of said MgO-yielding material beingless than 44 microns in size and having a median particle size of lessthan 4 microns, said MgO-yielding material containing less than 0.5% Si0on the ignited basis, with from 80% to 20% by weight of chrome ore atleast of which passes a 325 mesh screen and 50% of which is finer than10 microns, said chrome ore containing less than 3% SiO (2) subjectingsaid admixture without prior compaction to a heat treatment wherein theadmixture is exposed to a temperature of at least 1975 C. for at least10 minutes, and (3) recovering a refractory grain comprising a uniformpericlase solid solution with exsolved chrome-containing spinel withinindividual periclase crystallites, said grain having less than 10 volumepercent porosity and being substantially free of any of the originalchrome ore in unreacted form.

2. Method according to claim 1 wherein said admixture is exposed to amaximum temperature of at least 2000 C.

3. Method according to claim 1 wherein said MgO- yielding material ismagnesium hydroxide.

4. Method according to claim 3 wherein said admixture is exposed to amaximum temperature of at least 2000 C.

5. Method according to claim 3 wherein said magnesium hydroxide is theproduct of reaction between Ca0- containing material and brinecontaining soluble magnesium values.

6. Method according to claim 5 wherein said admixture is exposed to amaximum temperature of at least 2000 C.

References Cited UNITED STATES PATENTS Austin et al. l0659 Davies l0659Mikami 106--59 Neely l0659 Treffner l0659 JAMES E. POER, PrimaryExaminer

